Foaming is a common problem in many industrial processes, particularly in paper making operations where foam can prevent proper formation of the finished paper and disrupt manufacturing operations. Defoamers, sometimes called anti-foaming agents, can be added to process streams to combat this problem. Commonly used agents are insoluble oils, polydimethylsiloxanes and other silicones, certain alcohols, stearates and glycols. The defoamer can be added either to prevent formation of foam or to break a foam already formed. (Rainer Höfer et al., “Foams and Foam Control,” Ullmann's Encyclopedia of Industrial Chemistry, 2000, Wiley-VCH, Weinheim).
Experiments with silicone-based defoamers started during the 1950s. These were based on polydimethylsiloxane (silicone oil) dispersed in water or light oil. Silicone oils worked well, but caused surface disturbances in many applications like paints and papermaking. In 1963, the first defoamers made with hydrophobic silica particles in light oil were patented, including dispersions of solid particles in silicone oil (so-called “filled silicone”). These eventually were replaced by water-based defoamers, and today the predominant defoamers in the paper-making industry use filled silicone present as a silicone-in-water emulsion. The silicon itself is typically a dispersion of solid particles in one or more silicone fluids. This dispersion can employed as is, without further modification, mixed with other silicone oils, or emulsified in water, for defoaming or preventing foaming in liquid media.
These prior art silicone-in-water emulsions are commonly prepared by mixing silica in a methyl-terminated low viscosity siloxane at temperatures ranging from 140° C.-160° C. for a few hours before cooling. This mixture can later be homogenized under high shear to break apart the silica agglomerates to an evenly dispersed target particle size. A promoter can be added to promote the hydrophobization of any hydrophilic sites on the silica exposed during the homogenization process. Additional silicones can be added before or after the silica is dispersed and homogenized, typically of much higher viscosity, to increase the defoaming performance and longevity of the composition.
The manufacture of silicone-based defoamers is a time and heat intensive process, primarily because of the need to melt and mix the defoamer ingredients before forming an emulsion, and the sheer force required to homogenize silica in the silicone at a desired uniform particle size. What is needed are improved techniques and compositions for manufacturing silicone emulsion defoamer compositions that require less time and use less energy for production. Ideally, the resulting emulsion would have improved anti-foaming activity, improved finished product performance, and improved activity in the process stream to which it is added.